1. Field of the Invention
The present invention generally relates to an electrophotographic copying machine and, more particularly, to a charge eraser designed to erase by illumination at least a predetermined portion of the electrostatic charge built up on a photoreceptor drum in the copying machine.
2. Description of the Prior Art
The electrophotographic copying machine now available in the market generally has an charge eraser disposed in the vicinity of the photoreceptor drum and positioned upstream of the developing station with respect to the direction of rotation of the photoreceptor drum. This type of charge eraser is used to illuminate at least partially the photosensitive surface of the photoreceptor drum to remove a charged portion, before the electrostatic latent image formed by exposing the photosensitive surface to image rays that correspond to an image to be copied is developed into a toned image. Thereby an unwanted portion of the electrostatic charge built up on the photosensitive surface will be prevented from being electrostatically coated with toner particles. The unwanted portion of the electro-static charge to be removed or erased by illumination has long been chosen as a peripheral marginal portion of a copying paper generally outside the region of the latent image, and the charge eraser has been so designed and so positioned for this purpose.
On the other hand, demands have been made for the copying machine to have a capability of erasing not only the unwanted portion of the electrostatic charge, but also a portion of the electrostatic charge within the region of the latent image so that an operator of the copying machine can enjoy such effects as partial erasing, trimming, and composite copying. For example, given a certain image of particular size, the partial erasing will result in a blank left within a selected area of the image; the trimming would result in reduction in size of the image by the removal of an unwanted marginal area of the image; and the composite copying would result in the formation of a "window" on a selected area of the latent image for accommodating a different image which will be subsequently copied on the partially erased area of the image.
In order to meet the demands, the use of a charge eraser has been made, which eraser comprises a generally linear array of light emitting elements such as, for example, light emitting diodes, arranged in the vicinity of the photoreceptor drum so as to extend parallel to the axis of rotation of the photoreceptor drum. The light emitting elements are controlled by a matrix drive circuit so as to be selectively energized to emit rays of light to remove, in a predetermined pattern, a portion of the electrostatic charge on the photoreceptor drum.
Apart from the capability of making a window on the electrostatic charge built up on the photosensitive surface of the photoreceptor drum, the provision has been desired of a capability of imprinting the date, symbols and/or a brief commentary on the copying paper together with the copied image. This could possibly be accomplished if the light emitting elements are selectively energized to delineate alphanumeric characters with light beams so that portions of the electrostatic charge corresponding in pattern to the alphanumeric characters can be formed on the photosensitive surface of the photoreceptor drum.
Where the electrophotographic copying machine is to be fabricated having these two capabilities, care is required because the preciseness to which design requirements have to be fulfilled differs between these two capabilities. This will be discussed in detail with particular reference to FIG. 15 of the accompanying drawings, which illustrates the prior art charge eraser in fragmentary representation.
Referring to FIG. 15, the prior art charge eraser comprises a generally rectangular housing having a row of chambers isolated by partitions and accommodates therein respective light sources, such as light emitting diodes. According to the prior art, the charge eraser is so designed and so supported that the front of the housing with respect to the direction of projection of rays of light from the respective light sources may be spaced about 3 mm from the photosensitive surface of the photoreceptor drum. This is because, if the front of the housing is too close to the photosensitive surface of the photoreceptor drum so as to overlap rays of light emitted from the neighboring light sources with each other, the intensity of light projected from one or more of the LED light sources onto the photosensitive surface may vary from place to place on the photosensitive surface, accompanied by corresponding variation of the surface potential on the photosensitive surface, i.e., the potential of one or more portions of the electrostatic charge which are required to be removed from the photosensitive surface. The variation in potential of the photosensitive surface would not result in the eventual reproduction of a high quality image on the copying paper.
The selection of about 3 mm for the spacing between the front of the housing and the photosensitive surface of the photoreceptor drum is base on the utilization of interference of light emitted from the neighboring members of the LED light sources thereby to minimize the variation of the potential of the photosensitive surface of the photoreceptor drum.
It has, however, been found that, when about 3 mm is chosen for the spacing, the sharpness of the boundary between the erased area and the non-erased area in the electrostatic charge built up on the photosensitive surface will be impaired even though the problem associated with the varying surface potential on the photosensitive surface can be successfully eliminated. This in turn brings about a problem in that, when, for example, a certain character is to be imprinted, the character electrophotographically reproduced on the copying paper might be blurred as shown in FIG. 14(b).